Methods of improving the yield in alkaline digestion of cellulosic material



Patented Juiy 3,. 1982 Nils Josef Christoiicr Hartier, L. h1g0, Sweden, a 20:

economic considerations. However, the effect of the reducing agent is not increased above a certain limit, and suitably, therefore, the amount used will be below 3% by weight of the lignocellulosic material.

W: 1 n v WW6 k i P Sm 4.02m The digestion is otherwise carried out by conventional to mute-5 gi tgi methods, which need not be discussed in detail here, 55 Sen NIL 852,964 since they are well-known to those skilled in the art. The

ciaim's 'a Estimation Swedm NO 24, 1953 dlge stion is generally carried out at temperaturesof 160- 53 3 35. (Cl. g 200 C. By way of example, the process of 11115 1nven- 10 tron can also be applied to the digestion of a lignocellu- The present invention relates to alkaline digestion of losic material which prior to digestion has been subjected lignocellulosic materials, such as wood chips. The main t pr hmlnary treatment with digestion liquid f an object of the invention is to improve the yield of pulp lmtlal pH between 1 and 11 and a temperature between from the digestion, but also to provide qualitative im- 120 and 180 C. provernents of the resulting pulp, such as improved bright- 15 h l found. d 11118 forms anotnen as set of ness and beatability. As used herein, the term alkaline thlS tio l the treatment Vlth reducing agents digestion" refers mainly to h lf t nd soda processes of the above-mentioned type can advantageously be carof digesting lignocellulosic materials but other methods fled 011K011 1 g Shifting material. usually in which digestion takes place at least partly at a pH Wood lP Thls has The j 1 2 that cafhonyl; groups above 7, eg the so-called neutral sulfite process are with- Pfesfiht the Cellulose. Whlch form sfarhhg p f for in the scope f the invention degradation of the cellulose molecule in an alkalme en- It is known from a number of published investigations vll'ohmehh h ehmlnatefl l e y before the material is that the solubility in hot alkali of a cellulosic material conflicted With the alkaline s h '-l diminishes after it has been treated with a suitable oxi- It has further been found that t 18 re preferable dizing or reducing agent. not only to carry out pretreatment with a reducing agent,

As a suitable oxidizing agent, sodium chloride has been but 11159 to add a fedhclhg g digestion. Suitmentioned (0. Samuelson and N. Hartler, Svensk Kemisk y h g e period When the mixture of hgnocellulpsie Tidskrift. 62 1950 197 and this result has later been material and dlgestwn chemwals ght to the desired confirmed and studied more closely by several researchers mahlmum tfimpel'ahlre; lf Such Carbonyl gTOUPS (A. Meller. TAPPI 34 (1951), 171, and ibid. 35 (1952), Whwh are formed r s t g e are eliminated. 72; H. Richtzenbain et al., Svensk Papperstidning 57 The Pretreatment 1S Suitably camed at a p (1954), 363, and ibid. 57 (1954), 538). This OXidiZing ature wlthin the range of 0-100 (2. As in the addition agent seems to have a selective action. The carbonyl of q s agent to the {hgeshoh q the amount of groups of the cellulose material are oxidized to form reduclhg agent should sultahlyhfi t least 0.00l% and carboxvl groups. Other oxidizing agents tested (e.g. prefhrahly at least (101% by h 0f 909 1 010510 chlorine dioxide) have little or no effect in the respect starhhg mammals and for hh l reasons it is preferhere contemplated, i.e. to reduce the hot alkali solubility ifl l about 3% y Welghi 0f the g o el ulosic mat d' ute solution of sod i i 1 252532 ii g s-g gii :f to 100 m 120 C The invention is illustrated but not limited by the fol- As a suitable reducing agent to achieve the same effect, 40 lowmg examplessodium borohydride has been mentioned (A. Meller, 5x41711718 1 TAPPI 36 366, and 38 0- In this Sulphate cooking has been performed in autoclaaes case, too, a selective action is involved. The carbonyl i h a charge of 70 bone dry pine chips and with 3 groups of the celulose material are reduced to form alcowoodgiquor ratio f The alkalinity was 21% and hol Q P and the increased stability to alkali decreases the sulfidity 25%. No addition of black liquor was done. the hot alkali Solubility of the material- The liquor was added at C. and the temperature in- In one p the Present invention relates to the F creased to 170 C. during 2 hours and kept constant duress of adding in alkaline digestion of lignocellulosic maj 2 h A arying amount of sodium borohydride tefials, y soda of sulfate Process, a l'educlhg (02.l% calculated on the wood) was added to the ag which under the conditions prelaihhg the dlges' 50 cooking liquor prior to charging. The obtained pulps reduces Carbonyl groups of calhllose, whereby The were washed and screened and yield, Roe number, brightyield of p p is enhanced and the quality of the P is ness and strength properties were determined. The beatimproved in various respects. ing was performed in a PFI-mill. The following results Suitable reducing agents for this purpose are hydrides, were obtained.

Sodium Yield of uu-' i Cooking g gg gg Roe ness, Boating Slow- Breaking Bursting Tear {Double N0. (mated on numculatedon percentlreyoluness, length, strength factor i told the woo be! the wood) G tlolls SR km. km./cm.-i I percent percent I g o 4.3 40.2 32 18.000 41[ 0.9 s.1 160; 6,700 0 7 4.3 50.3 35 18,000 52 10.6 8.8 11, 500 2 1 4.4 I 52.1 35 18, 000 52 i 10.0 5.2 1315 5 s, 500

such as alkali metal borohydrides and alkali metal aluminium hydrides, e.g. sodium borohydride, potassium borohydride and lithium aluminium hydride. Gaseous hydrogen can also be utilized.

The amount of reducing agent used should suitably be at least 0.001% and preferably at least 0.01% by weight of the lignocellulosic material. Upwardly the amount of reducing agent to be used is limited only by As is seen from the table an addition of sodium borohydride increases the yield at constant degree of delignification from 46.2% without an addition to 50.3% respectively 52.1% with an addition of 0.7 respectively 2.1% of sodium borohydride corresponding to an increase in yield of 8.9% respectively 12.8%. It can furthermore be seen that an addition of sodium borohydride increases the brightness as well as the beatability of the pulp.

Example 2 Soda cooking was performed in autoclaves with a charge of 70 g. bone dry spruce chips and a wood liquor ratio of 1:4. The amount of added alkali was 30% sodium hydroxide calculated on the wood. No addition of black liquor was made. The liquor was charged at a temperature of 70 C. and the temperature was increased to 170 C. during 2 hours and was kept constant at that temperature during 2 hours. Varying amount of sodium borohydride (-2.l% calculated on the wood) was added to the cooking liquor prior to charging. The obtained pulp was washed and screened and yield and Roe numher were determined. The following results were obtained.-

Sodium Yield of unborohydridc bleached pulp Cooking No. (calculated Roe number (calculated on the wood), on the wood), percent percent 0 6. s I 42.1 0. 7 0. 2 45. 9 2.1 6. 1 47. s

As is seen from the table an addition of sodium borohydride increases the yield at constant degree of delignification.

' Example 3 Sulphate cooking has been performed in autoclaves with a charge of 70 g. bone dry pine chips and with a wood liquor ratio of 1:4. The alkalinity was and the sulfidity No addition of black liquor was done.

Sodium Yield of unhorohydride bleached pulp Cooking No. (calculated Roe number (calculated on the wood), on the wood), percent percent 0 4. 1 45. 6 0. 2 4. l 45. 6 0. 4 4. 0 47. O 0. 6 3. 7 4S. 6 0. 8 4. 2 49. 3

As is seen from the table an addition of sodium borohydride to the wood in an impregnation stage increases the yield at a constant degree of delignification.

Example 4 Sulphate cooking was performed in autoclaves with continuous liquor flow. The wood charge was g. bone dry pine chips and the cooking liquor had a flow speed of 2 liter/hour. The cooking liquor contains 10 g. sodium hydroxide and 10 g. sodium sulphate per liter. In some cases the wood was pretreated in an impregnation stage termined. The following results were obtained.

NQBII; Yield of in the Na'Bll; 11n- Impregcooking in the .slcached Cooking No. nation liquor cooking Roe pulp (calwith during liquor number eulated NaB H tempeat- 170 C. on the rnture \OMD, increase percent As is seen from the table an addition of sodium borohydride to the wood in an impregnation stage results in increased yield at unchanged degree of delignification as far as sulphate cooking is concerned. The increase in yield is, however, greater if sodium borohydride also is added during the temperature increase period of the cook. The increase in yield isapproximately 4.5% calculated on the wood when sodium borohydride is present only in the impregnation stage but when sodium borohydride is present during the impregnation stage as well as during temperature increase period of the sulphate cook the increase in yield is approximately 8%, i.e. an additional 3.5%. This is true when the cook is performed to a Roe number of 7 whereas another distribution of the increase in yield is obtained when the cooking is performed to a lower Roe number as is also seen from the table.

I claim:

l. In the preparation of pulp for paper-making by the digestion of wood chips in an alkaline medium at a temperature of between and 200 C., the improvement which comprises carrying out the digestion in the presence of a reducing agent selected from the group consisting of hydrogen and metal borohydrides.

2. The method as set forth in claim 1, in which the reducing agent is added during the temperature increase period of the digestion.

3. The method as set forth in claim 1 in which the reducing agent is an alkali metal borohydride.

4. The method as set forth in claim 1 in which the reducing agent is sodium borohydride.

5. The method as set forth in claim 3 in which the amount of the reducing agent is between 0.01% and 3.00% by weight of the wood chips.

I 38, No. 11', November 1955, pages 682-687. 

1. IN THE PREPARATION OF PULP FOR PAPERMAKING BY THE DIGESTION OF WOOD CHIPS IN AN ALKALINE MEDIUM AT A TEMPERATURE OF BETWEEN 160* AND 200*C., THE IMPROVEMENT WHICH COMPRISES CARRYING OUT THE DIGESTION IN THE PRESENCE OF A REDUCING AGENT SELECTED FROM THE GROUP CONSISTING OF HYDROGEN AND METAL BOROHYDRIDES. 